The present invention relates to a zoom lens system in which compensation for aberrations is kept in superior condition when a lens group is added to or removed from a portion in the lens system for varying the magnification.
Heretofore, when the telefoto range is desired to be enlarged by extending the focal length of the main photographing lens, it has been conventional practice to utilize either a teleconvertor (afocal telefoto lens system) or an extender.
And, a lens system was well known as disclosed in U.S. Pat. No. 2,854,891 in which a teleconvertor is provided in front of the objective lens without having the zooming operation.
As a lens system used in a television camera, however, a zoom lens having the zooming ratio in the degree of 10.times. has been used as the standard lens system. Therefore, the diameter of the foremost lens element of the lens system will reach 100 - 150 mm and the weight of the lens system also amounts to more than 10 kg. The teleconvertor to be attached to such a zoom lens will have too large a lens element diameter and the weight of the lens system will be too great to be conveniently operated.
Therefore, an extender shown as 7 in FIG. 2 has been generally utilized in the viewpoint of easy manipulation as well as the size of the lens system. However, minor deterioration in the performance can not be avoided. Further, in the case of a color television camera, a prism optical system 3 for separation of three color channels of red, green and blue is provided in the camera body 1 behind the objective lens 2 at the back focus point thereof as shown in FIG. 1. Therefore, when it is desired to use the extender, it is necessary to detach the objective lens 2 from the camera body 1 as shown in FIG. 1 and to attached the extender 7 to the camera body 1 followed by the remounting of the objective lens 2 thereon as shown in FIG. 2.
In this case, if the flange backs of both the objective lens and the extender are exactly provided with sufficient accuracy, readjusting of focus will not be required. However, extremely high mechanical accuracy is required in the parts to achieve such a condition thereby making it impractical. Therefore, it is practically necessary to effect readjustment of focus or tracking for the respective camera pickup tubes 4, 5 and 6 for red, green and blue, and prolonged troublesome operation is required for remounting of the zoom lens, because the weight of the zoom lens as described above amounts to 10 kg.